Covered metals are used in a wide variety of technical applications. The purpose of the covering is to improve the working properties of the product such as, for example, the generalized corrosion resistance, the fatigue strength, the electrical contact resistance, the thermal insulation etc. By way of example and without limitation, products which may be mentioned are galvanized steels, cadmium-plated bolts, and ferromagnetic conductive products covered with a film of silver or gold or another precious metal.
In order to guarantee that the covering skin has the desired effect on the working properties of the product, it must be possible to accurately check the characteristics of the coating giving rise to the desired effect. It is necessary, for example, to be capable of accurately determining the thickness of the covering skin, it being possible for the latter to vary from very small values (for example 1/10 or 1/100 of a micron) to relatively large values (for example 1 mm). The electrical conductivity .sigma. must sometimes also be determined. Finally, it is sometimes also necessary to check the uniformity of the covering layer and, where appropriate, its integrity. It is advantageous if all these measurements can be carried out without contact with the article, for example in order to make it possible to introduce these inspection techniques on-line into the production process.
Numerous methods have been developed and applied for measuring the thickness of a layer deposited on a conductive support.
There are known chemical industrial methods for determining the thickness of a coating on a product. It should be noted that these methods have the major drawback that they are destructive and therefore cannot be applied on-line on production lines.
Other methods, such as the use of ultrasound or measuring the absorption or diffraction of ionizing radiation have been proposed for determining the thickness of a thin layer deposited on a substrate.
Other methods, carried out by measuring eddy currents, have also been described in the scientific literature or in patents.
In particular, the document "Thickness and Conductivity of Metallic Layers from Eddy Current Measurements", Rev. Sci. Instrum. 63 (6), June 1992, pp. 3455-3465 and the document FR-A-83 15 587 describe methods for measuring a non-ferromagnetic covering on non-ferromagnetic substrates.
The document "Utilisation de l'inversion de modeles courants de Foucault pour mesure de l'epaisseur de zinc sur aciers galvanises" Use of eddy-current model inversion for measuring the thickness of zinc on galvanized steels!, comptes-rendus de la 6.sup.eme Conference sur les controles non destructifs, Proceedings of the 6th conference on non-destructive testing! Nice, October 1994, pp. 1325-1329 and patents EP-A-0 179 720 and FR-A-83 12 980 describe methods and devices for measuring the thickness of thin metallic layers deposited on a conductive support.
However, all these methods are sensitive to the quality of the ferromagnetic conductive substrate, such as, for example, the conductivity and/or permeability.
Furthermore, in order to obtain reliable measurements, all cases require calibration to be carried out in order to make measurement possible.
All the documents referred to above describe methods which, during the measurement, employ only a single data item, such as, for example, the inductance, resistance or else the angle between the inductance and the resistance.
All existing methods for measuring the thickness of conductive layers coating conductive substrates therefore have drawbacks which limit their field of application, which cause measurement inaccuracies and which, when the methods are applicable, require very stringent checks that the geometrical, electrical and magnetic characteristics of the material are invariant. Rigorous control of the constancy of these characteristics is virtually impossible in an industrial environment.